It means that the walkmeshes for those placeables are missing. For most of them that doesn't matter because placeables like carpets, visual effects, etc shouldn't block movement anyway. For others it is a real error but one that has been there for years. You can for example walk through corpse placeables. Those are so big that they should have a pwk (a walkmesh for placeables) but they haven't got one and never had and now people are probably so used to being able to walk through those corpses that nobody would like this to be fixed.
Invalid use nodes are use nodes placed on a part of the placeable that can't be reached because it isn't walkable. Those placeables probably had two use nodes so players could use them from two different sides and then were imported into (g)max and one use node got moved to 0,0,0 because there's a bug in the import scripts that sometimes moves one of the use nodes to 0,0,0 if there are two use nodes.
One of the use nodes will still work so it's not a bug that is obvious in game.